Supposing your text is displayed in a label or so you get the position on this label and with QFontMetrics you can calculate the metrics of this text to get the position of single words.
If you have a label only containing "signal" it is enough to know that you are hovering the label.

the word could be in any place of a screen , a web page ,a PDF page and so on .I just want to find a generic way to get the world the mouse cursor pointing at .
I also thought of OCR ,but this technology is not mature enough !